Aircraft pilots are expected to visually identify collision threats and avoid them. This "see and avoid" technique based on the pilot's visual sense remains the most basic method of aircraft collision avoidance. However, since the 1950's electronic techniques based on radio frequency and optical transmissions have been developed to supplement the pilot's visual sense. The government has developed and implemented a system of ground based and aircraft carried equipment designated the Air Traffic Control Radar Beacon System (ATCRBS). This system includes two different types of ground based radar emitters located at each of a plurality of Air Traffic Control (ATC) stations. One type of radar is referred to as the Primary Surveillance Radar (PSR), or simply as the primary radar. The primary radar operates by sending out microwave energy which is reflected back by the aircraft's metallic surfaces. This reflected signal is received back at the ground radar site and displayed as location information for use by an air traffic controller. The second type of radar is referred to as the Secondary Surveillance Radar (SSR), or simply secondary radar. Unlike the primary radar, the SSR is a cooperative system in that it does not rely on reflected energy from the aircraft. Instead, the ground based SSR antenna transmits a coded 1030 MHz microwave interrogation signal. A transponder, i.e., a transmitter/receiver, carried on the aircraft receives and interprets the interrogation signal and transmits a 1090 MHz microwave reply signal back to the SSR ground site. This receive and reply capability greatly increases the surveillance range of the radar and enables an aircraft identification function, referred to as Mode-A, wherein the aircraft transponder includes an identification code as part of its reply signal. This identification code causes the aircraft's image or blip on the ATC operator's radar screen to stand out from the other targets for a short time, usually about 20 seconds. Thus, Mode-A provides an rudimentary identification function.
In addition to the identification function provided by Mode-A, the aircraft altimeter is typically coupled to the transponder such that a reply signal includes altitude information, referred to as Mode-C.
A ground based SSR sequentially transmits both Made A and Mode-C interrogation signals to aircraft in the area. Accordingly, the interrogation signal transmitted by the SSR contains three pulses. The second pulse is a side-lobe suppression signal transmitted from an omnidirectional antenna co-located with a mechanically rotating antenna which provides a highly directive antenna beam. The first and third pulses are transmitted by the directive antenna at a predetermined frequency and are separated by a predetermined interval. The time interval between the first and third pulses defines what information the interrogator is requesting: eight (8) microseconds for identification and twenty-one (21) microseconds for altitude. The operator of the ground based SSR sets the radar interrogation code to request either Mode-A or Mode-C replies from the aircraft transponder. Typically, the radar is set to request a sequence of two Mode-A replies followed by a single Mode-C reply. This sequence is repeated so that a radar operator continuously receives both the Mode-A identification code and the Mode-C altitude information. Upon receipt of the interrogation signal, the aircraft transponder develops and transmits a reply signal which includes the identification or altitude information. The ground based SSR receives and processes the transponder reply signal, together with time of arrival range information, to develop a measurement of position for each responding aircraft. Under such a system, the air traffic controller uses this information to contact involve the aircraft by radio, usually with voice communication, to maintain or restore safe separations between aircraft. The system is inherently limited because each aircraft needs be dealt with individually which requires a share of the air traffic controller's time and attention. When traffic is heavy, or visibility is low, collision potential increases.
During the 1960's the increases in the number of aircraft, the percentage of aircraft equipped with transponders, and the number of ATCRBS radar installations began to overload the ATCRBS system. This system overload caused a significant amount of interference and garble in the Mode-A and Mode-C transmissions because of replies from many simultaneously interrogated aircraft. Furthermore, the Mode-A and Mode-C systems are unable to relay additional information or messages between the ground based SSR and the interrogated aircraft, other than the aforementioned identification and altitude information. The Mode Select, or Mode-S, was the response to this overload and other deficiencies in ATCRBS. Mode-S is a combined secondary surveillance radar and a ground-air-ground data link system which provides aircraft surveillance and communication necessary to support automated ATC in the dense air traffic environments of today.
Mode-S incorporates various techniques for substantially reducing transmission interference and provides active transmission of messages or additional information by the ground based SSR. The Mode-S sensor includes all the essential features of ATCRBS, and additionally includes individually timed and addressed interrogations to Mode-S transponders carried by aircraft. Additionally, the ground based rotating directive antenna is of monopulse design which improves position determination of ATCRBS target aircraft while reducing the number of required interrogations and responses, thereby improving the radio frequency (RF) interference environment. Mode-S is capable of common channel interoperation with the ATC beacon system. The Mode-S system uses the same frequencies for interrogations and replies as the ATCRBS. Furthermore, the waveforms, or modulation techniques, used in the Mode-S interrogation signal were chosen such that, with proper demodulation, the information content is detectable in the presence of overlaid ATCRBS signals and the modulation of the downlink or reply transmission from the transponder is pulse position modulation (PPM) which is inherently resistant to ATCRBS random pulses. Thus, the Mode-S system allows full surveillance in an integrated ATCRBS/Mode-S environment.
The Radio Technical Commission for Aeronautics (RTCA) has promulgated a specification for the Mode-S system, RTCA/DO-181A, Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode-S) Airborne Equipment, issued January 1992, and incorporated herein by reference. According to RTCA specification DO-181A, the airborne portion of the Mode-S system includes in one form or another at least a dedicated transponder, a cockpit mounted control panel, two dedicated antennas and cables interconnecting the other elements. Shadowing is attenuation of the received transponder signals by the airframe blocking the antenna from the SSR ground station transmitter when a single antenna is used. The shadowing problem is overcome by locating a first antenna on a top surface of the aircraft and a second antenna on a bottom surface of the aircraft. As discussed more fully below, each aircraft may be within range of more than one SSR ground station at any time and must respond to interrogation signals broadcast from multiple directions. Therefore, the Mode-S system uses two single element omnidirectional antennas to receive interrogation signals from any quarter and reply in kind.
In operation, a unique 24-bit address code, or identity tag, is assigned to each aircraft in a surveillance area by one of two techniques. One technique is a Mode-S "squitter" preformed by the airborne transponder. Once per second, the Mode-S transponder spontaneously and pseudo-randomly transmits (squits) an unsolicited broadcast, including a specific address code unique to the aircraft carrying the transponder, via first one and then the other of its two dedicated antennas which produce an omnidirectional pattern, discussed below. The transponder's transmit and receive modes are mutually exclusive to avoid damage to the equipment. Whenever the Mode-S transponder is not broadcasting, it is monitoring, or "listening," for transmissions simultaneously on both of its dedicated omnidirectional antennas. According to the second technique, each ground based Mode-S interrogator broadcasts an ATCRBS/Mode-S "All-Call" interrogation signal which has a waveform that can be understood by both ATCRBS and Mode-S transponders. When an aircraft equipped with a standard ATCRBS transponder enters the airspace served by an ATC Mode-S interrogator, the transponder responds to the with a standard ATCRBS reply format, while the transponder of a Mode-S equipped aircraft replies with a Mode-S format that includes a unique 24-bit address code, or identity tag. This address, together with the aircraft's range and azimuth location, is entered into a file, commonly known as putting the aircraft on roll-call, and the aircraft is thereafter discretely addressed. The aircraft is tracked by the ATC interrogator throughout its assigned airspace and, during subsequent interrogations, the Mode-S transponder reports in its replies either its altitude or its ATCRBS 4096 code, depending upon the type of discrete interrogation received. As the Mode-S equipped aircraft moves from the airspace served by one ATC Mode-S interrogator into that airspace served by another Mode-S interrogator, the aircraft's location information and discrete address code are passed on via landlines, else either the ground based SSR station picks up the Mode-S transponder's "squitter" or the Mode-S transponder responds to the All-Call interrogation signal broadcast by the next ATC Mode-S interrogator.
The unique 24-bit address code, or identity tag, assigned to each aircraft is the primary difference between the Mode-S system and ATCRBS. The unique 24-bit address code allows a very large number of aircraft to operate in the air traffic control environment without an occurrence of redundant address codes. Parity check bits overlaid on the address code assure that a message is accepted only by the intended aircraft. Thus, interrogations are directed to a particular aircraft using this unique address code and the replies are unambiguously identified. The unique address coded into each interrogation and reply also permits inclusion of data link messages to and/or from a particular aircraft. To date, these data link messages are limited to coordination messages between TCAS equipped aircraft, as discussed below. In future, these data link messages are expected to include Aircraft Operational Command (AOC) information consisting of two to three pages of text data with flight arrival information, such as gates, passenger lists, meals on board, and similar information, as well as Flight Critical Data (FCD). However, the primary function of Mode-S is surveillance and the primary purpose of surveillance remains collision avoidance.
Collision avoidance systems which depend on aircraft carried transponders are usually divided into two classes: passive and active. The ATCRBS, including Mode-S, described above are passive systems because the transponder reply emissions alone provide the only information for locating and identifying potential threats. While passive systems tend to be simple and low cost when compared to active systems and do not crowd the spectrum with additional RF transmissions, detection of transponder emissions from other aircraft is difficult. A passive collision threat detector is essentially a receiver having sufficient intelligence to first detect and then locate the existence of potential collision threats represented by nearby aircraft. The aircraft's receiver is of necessity operating in close proximity to the host aircraft's ATCRBS transponder. Government regulations require the ATCRBS transponder to emit RF energy at 125-500 watts in response to interrogation signals from a ground based SSR. The transponder aboard any potential collision threat aircraft flying along a radial from the directional SSR antenna, usually about 3.degree. to 4.degree. wide, will respond at about the same time as the host aircraft's transponder. The host aircraft's transponder is so much closer, usually no more than a few feet, to any receiver that the host aircraft's own response to the interrogation signal will swamp the response from any other aircraft in its vicinity. Thus, the host aircraft flies in a "blind" region wherein any potential threat aircraft is not "seen," unless other provisions are made. This blind region expands as the target approaches the host. Furthermore, typically each aircraft is within range of more than one SSR site and a blind region is associated with each SSR site. Because wholly passive systems are generally believed insufficient for reliable collision avoidance, the government and aviation industry have cooperated in developing Operational Performance Standards for a Traffic Alert and Collision Avoidance or TCAS system, separate from the ATCRBS/Mode-S transponder system. The standards are set forth in the RTCA specifications DO-185, Minimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, issued Sep. 23, 1983, consolidated Sep. 6, 1990, and DO-185A, Minimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System II (TCAS II) Airborne Equipment, issued December 1997, both of which are incorporated herein by reference.
TCAS is a well-known active collision avoidance system that relies upon reply signals from airborne transponders in response to interrogation signals from an aircraft equipped with a ATCRBS Mode-A/Mode-C or Mode-S transponder. The TCAS antenna is driven to produce an omnidirectional microwave transmission, or radiation, pattern carrying a transponder generated coded interrogation signal at 1030 MHz, the same frequency used by ground based SSR stations to interrogate Mode-S transponders. Whenever the TCAS transponder is not broadcasting, it is "listening" for Mode-S "squitters" and reply transmissions at 1090 MHz, the same frequency used by Mode-S transponders to reply to interrogation signals. Thus, a TCAS equipped aircraft can "see" other aircraft carrying a transponder. Once a transponder equipped target has been "seen," the target is tracked and the threat potential is determined. Altitude information is essential in determining a target's threat potential. Comparison between the altitude information encoded in the reply transmission from the threat aircraft and the host aircraft's altimeter is made in the TCAS processor and the pilot is directed obtain a safe altitude separation, by descending, ascending or maintaining current altitude.
Collision avoidance is enhanced by including range information during threat determination. The approximate range, or distance between the host aircraft and the target, is based on the strength of the received transponder signal in response to an interrogation signal from the host aircraft. Modern TCAS systems obtain more accurate range information by measuring the time lapse between transmission of the interrogation signal and reception of the reply signal, commonly known as "turn around time." The time to closest approach as determined by the TCAS processor is the primary consideration in threat determination.
Knowledge of the direction, or bearing, of the target aircraft relative to the host aircraft's heading greatly enhances a pilot's ability to visually acquire the threat aircraft and provides a better spatial perspective of the threat aircraft relative to the host aircraft. The TCAS processor can display bearing information if it is available. Bearing information is also used by the TCAS processor to better determine threat potential presented by an intruder aircraft. Directional antennas are used in some TCAS systems for determining angle of arrival data which is converted into relative bearing to a threat aircraft by the TCAS processor. Several methods exist for determining angle of arrival data. One common arrangement uses a phase matched quadrapole antenna array with output signals being combined such that the phase difference between two output ports of the combining circuitry indicates the bearing of a received transponder signal. Another method for determining angle of arrival data include a method based on signal phase, commonly known as phase interferometry. Still another commonly known method is based on signal amplitude. Attenuation of the received transponder signals by the airframe blocking the antenna from the transmitter is often overcome by locating a primary directional antenna on a top surface of the aircraft and a second antenna on a bottom surface of the aircraft. The second or bottom antenna is sometimes omnidirectional which reduces cost at the expense of reduced directional coverage. Other TCAS systems provide duplicate directional antennas top and bottom. U.S. Pat. No. 5,552,788, Antenna Arrangement And Aircraft Collision Avoidance System, issued Sep. 3, 1996, the complete disclosure of which is incorporated herein by reference, teaches an arrangement of four standard monopole antenna elements, for example, 1/4 wavelength transponder antennas, arranged on opposing surfaces of one axis of the aircraft at the extremes of two mutually orthogonal axes to avoid shadowing and provide directional information about the received reply signal. For example, two monopole antennas are preferably mounted on a longitudinal axis of the aircraft and two additional monopole antennas are preferably mounted on a lateral axis of the aircraft orthogonal to the longitudinal axis passing through the first two antennas. Directionality is determined by comparing the power levels of the received signals. Additionally, the '788 patent teaches a TCAS system which can transmit transponder interrogation signals directionally using predetermined ones of the monopole antennas, thus eliminating dependence upon ground based radar systems for interrogating threat aircraft transponders.
Other antennas for directionally transmitting TCAS system transponder interrogation signals are also commercially available. For example, a TCAS system-compatible directional antenna is commercially available from AlliedSignal Incorporated of Redmond, Wash., under the part number ANT 81A.
Although the ATCRBS/Mode-S surveillance system and the TCAS collision avoidance system are separate, the TCAS processor accounts for the data provided by the intruder aircraft to determine what evasive maneuver to recommend to the host aircraft's pilot, i.e., whether to recommend that the pilot maintain current altitude, ascend or descend. The TCAS system also uses the inter-aircraft data link provided by the addressable Mode-S transponder to coordinate the recommended evasive maneuver with a TCAS equipped intruder aircraft. Furthermore, a connection between the TCAS and Mode-S transponders and other avionics on an aircraft allows coordination between the TCAS and Mode-S transponders. This intersystem connection is often used to prevent simultaneous transmissions which could interfere with the system's independent functions or cause equipment damage.
As briefly described above and described in detail in the respective RTCA specifications, DO-181A and DO-185A, the ATCRBS/Mode-S surveillance and TCAS collision avoidance systems are separate. The most basic installations require at least a TCAS processor, a Mode-S transponder, and two sets of independent and dedicated antennas. For example, U.S. Pat. No. 5,077,673, Aircraft Traffic Alert And Collision Avoidance Device, issued Dec. 31, 1991, describes a host aircraft having both an ATCRBS surveillance device and an aircraft traffic alert and collision avoidance device installed thereon, each of the ATCRBS surveillance device and an aircraft traffic alert and collision avoidance device having an antenna dedicated to supporting the respective independent function. U.S. Pat. No. 5,552,788 suggests using four dedicated monopole antennas to support just the an aircraft traffic alert and collision avoidance device. These redundant antennas are costly and add unnecessary weight to the aircraft. The omnidirectional nature of each of the Mode-S "squitter" and the Mode-S reply transmission require large amounts of transmission power and crowd the spectrum with additional RF transmissions, thereby degrading the RF interference environment. Although RTCA documents have suggested the possibility of a combined TCAS/Mode-S system, to date no enabling disclosure has been made and no product embodying such a combined TCAS/Mode-S system has been either used or offered for sale. Furthermore, no publication to date has suggested a combined TCAS/Mode-S system wherein both functions share common antennas.